Modern automobiles are equipped with a variety of control systems that improve passenger safety and comfort. Such systems include anti-lock brakes, differential steering and cruise control. Recently, GPS receivers have also been added to many cars for use in positioning purposes, allowing the driver to view his/her position on a map, and receive directions in real time. The advent of GPS receivers in cars has initiated efforts to enhance existing control systems, and to create new ones such as car-following and lane-keeping control. Many of these control systems require measurements of the vehicle's attitude [15].
Attitude determination by differential carrier-phase GPS has been studied extensively by researchers, and has resulted in numerous publications. However, few practical products have emerged from this research to date. The commercial attitude systems that do exist perform very well in environments where many GPS satellites are viewable with few obstructions, but not where buildings and trees cause frequent interruptions and outages of GPS signals. The traditional GPS attitude solution requires three or four satellites (depending on whether line bias is known) and depends on a continuous, integrated value of the carrier phase from multiple antennas; any momentary interruption in the signal tracking causes the integration to reinitialize (this is called a cycle slip). A cycle slip requires that the attitude-computation algorithm perform a new integer-ambiguity search, which is always susceptible to error, especially if some of the signals are noisy. A GPS attitude system mounted on a car often sees enough cycle slips and noisy signals to prevent convergence of integer searches, or worse, to allow convergence to wrong integer solutions. In addition to this performance shortcoming, commercial GPS attitude systems are prohibitively expensive (over $10K) for general automotive use. Accordingly, the art is in need of new developments of vehicle attitude systems.